If you still want to run flash in your web browser like before you can get the Basilisk web browser here.
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You'll probably need this as well to disable the killswitch on flash. It's the archived flash.
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You'll also need an ad blocker since the internet is cancer without one. Unfortunately I think only the legacy version works with Firefox so the results might not be as good as the current version but better than nothing right? Install the xpi file from this github.
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I'll mention for the hell of it but this is all some what insecure so if you want to run flash games in your browser you might want to run it in a virtual machine however if you are only visiting sites like newgrounds and other sites with good reputations I wouldn't go to the hassle of running it in a VM because these sites aren't hellbent on screwing you over.